Somatic cell hybrid cell clones prepared from Chineese hamster and mouse cells which contain different compliments of mouse chromosomes have been analysed for the presence of specific endogenous xenotropic env related DNA sequences. Continuing studies of the various somatic cell hybrid populations, by their orginator and others using classical virological and genetic techniques in concert with Mendelian genetic studies of the various donor mouse strains have permitted with mapping of some of the many genes involved in the expression of endogenous xenotropic MuLV. One of these, the Bxv-1 locus common to BALB/c, AKR, C47BL, C57L, and C58 mice has been mapped to chromosome 1 in each of these strains. Our current studies have shown that at least 3 endogenous xenotropic related HindIII fragments are localized on chromosome 1 and analysis of these fragments are compatible with one of them being the Bxv-1 inducible locus, one being a MCF precursor sequence and the third being a sequence not known to give rise to known MuLV. Further detailed studies of the DNA from the somatic cell hybrids in comparison to DNA from BALB/c mouse liver have allowed the assignments of at least 5 other endogenous xenotropic fragments to specific chromosomes.